This invention relates to vehicles that can be converted between four-wheel and two-wheel drive and more particularly to the manner of controlling the different modes of engagement.
It is common to provide vehicles with the capability to transfer or convert between four-wheel and two-wheel drive. A vehicle is typically provided with permanent rear wheel drive and the front wheels are provided with selective drive. The front wheels may be provided with a type of front wheel drive referred to as automatic four-wheel drive where the front wheels are driven only when the rear wheels overdrive the front wheels. They may instead or additionally be provided with a locked four-wheel drive where the rear wheels and front wheels are driven together under all conditions.
There are benefits and advantages of each mode of operation (two-wheel drive, automatic four-wheel drive and locked four-wheel drive). Two-wheel drive mode is the most efficient. The drive mechanism (propellor shaft, differential gears and axles) for the front wheels are rendered inactive and that drive mechanism does not consume power and does not wear when idle. Automatic four-wheel drive mode is the easiest for the driver. The driver does not have to forecast when four-wheel drive may be needed, it is not operational when it is not needed and it engages when needed. Locked four-wheel drive may be considered the safest and most reliable. It provides for engine braking of the front wheels and it always produces driving of all four wheels whether going forward or backward.
Numerous situations could be described wherein the driving conditions favor one form of drive mode over the others. The present invention is directed to a control feature that provides selection as between the different modes of drive. The control may be designed for manual selection and/or it may be designed for automatic selection of the desired drive mode (referred to sometimes as smart control).
Switching between two-wheel drive and automatic four-wheel drive is known and two systems for accomplishing this conversion are referred to as the pawl clutch system (see U.S. Pat. Nos. 5,927,455 and 5,967,277) and the roller clutch system (see U.S. Pat. No. 5,195,604). In both of these clutch systems a clutch mechanism is positioned between a drive and driven member and controlled by a control member. The control members have limited rotation relative to the drive member. The control members are frictionally engaged with xe2x80x9cgroundxe2x80x9d and are accordingly urged to a trailing position as permitted by said limited relative rotation. In the trailing position, the clutch mechanism inter-engages drive and driven members when the drive member attempts to overrun the driven member and does not produce inter-engagement of the drive and driven members when the driven member overruns the drive member. This is the automatic four-wheel drive mode of driving and functions the same whether driving forward or reverse. That is, upon reverse driving the trailing position of the control member is at the opposite end of the permitted relative rotation as between the control member and drive member and the clutch mechanism reverses in operation.
The present invention recognizes that if the control members become frictionally engaged to the driven member instead of to ground, the control member will still be urged to the trailing position of relative rotation when the drive member tries to overdrive the driven member (the same as in automatic four-wheel drive mode). However, the control member will also be urged to the opposite or leading position of relative rotation (as between the drive member and control member) if the driven member tries to overrun the drive member. The latter position is the same position as if the vehicle were driven in reverse and produces inter-engagement of the drive and driven members. Thus, the drive and driven members are locked together regardless of which member attempts to overrun the other which is the locked four-wheel drive mode.
It is also a characteristic of both the pawl clutch system and the roller clutch system that when the control member is substantially centered between the rearward most position and forward most position of permitted relative rotation with the drive member, the clutch mechanism provides no inter-engagement as between the drive and driven members. (There is an alternate arrangement of the pawl clutch system but the present invention utilizes that arrangement wherein the pawls are disengaged in the centered position.)
The present invention provides the capability of shifting the control member between positions of frictional engagement with ground, frictional engagement with the driven member and no frictional engagement. A centering mechanism urges centering of the control member between the rearward and forward most positions of relative rotation and when there is no frictional urging of the control member, the drive and driven members are free to rotate independently, which is the two-wheel drive mode.
Three different embodiments are disclosed herein for shifting the control member between the different modes of operation. The preferred embodiment utilizes a motor-driven lead screw that shifts a fork. The fork engages the control member and shifts the control member between a position of engagement with the driven member, engagement with ground and an intermediate position of non-engagement with either.
A second embodiment of the invention uses two electromagnets. An armature is in frictional engagement with the control member. The first electromagnet generates attraction of the armature and thus the control member to the driven member (locked four-wheel drive). The second electromagnet generates attraction of the armature to ground (automatic four-wheel drive). With both electromagnets deactivated, the armature rotates freely and the control member is urged by a centering spring to its center position (two-wheel drive).
A third embodiment uses a two-stage electromagnet and a return spring acting against an axially movable armature. The armature is rotatively coupled to the control member and the return spring urges the control member into frictional engagement with the driven member (locked four-wheel drive). Activation of the electromagnet generates a magnetic force that opposes the spring and draws the armature away from the driven member. The spring and electromagnet are cooperatively designed to provide one force level sufficient to draw the armature into frictional engagement with the coil which is grounded (automatic four-wheel drive). An intermediate magnetic force level draws the armature away from the control member which releases the frictional engagement with the driven member but not into frictional engagement with the coil. The centering mechanism then centers the control member for two-wheel drive.
Whereas the above refers to three different embodiments, it will be appreciated that each embodiment is applicable to either of the roller clutch system or the pawl clutch system. Furthermore, 46 additional systems may be devised and the invention may be incorporated into further embodiments of the invention as will be more fully appreciated upon reference to the following detailed description having reference to the accompanying drawings.